The present invention relates to a method for manufacturing a mold for continuous casting of high melting metals such as steel, the mold is to have rectangular and here particularly square-shaped cross section.
Molds of the type to which the invention pertains are usually made of copper or a copper alloy and carry a wear-resisting coating on the inside i.e. on the surface facing the molten material. Such a mold has to have a very high thermal conductivity, particularly when processing high melting metal such as iron and steel, so that the heat content of the molten material can be reduced as rapidly as possible. The wall thickness of such a mold has to be at least as large as is required for reasons of mechanical stability under particular consideration of mechanical loads in general.
Since copper has a very high thermal conductivity, its use for molds for continuous casting is preferred. On the other hand, the mechanical properties of copper are usually insufficient, and it is for this reason that recently, molds for continuous casting employ low alloyed copper alloys so that the mechanical properties of the mold can be improved. Such a copper alloy has, however, a somewhat reduced thermal conductivity, but in the overall balance, copper alloys constitutes a preferred material for making such molds.
It was found, however, that the continuous casting of certain kinds of steel using copper or copper alloy molds, dissolve some of the copper, i.e. some of the copper molecules are included in this steel resulting in a grain boundary diffusion and that, in turn, may lead to the infamous red shortness of the steel. For this reason, it has been proposed to cover the interior surface of the mold, i.e. that surface which engages the molten metal, with a wear resisting coating preventing the diffusion of copper molecules into the steel. Such a coating should increase the wear resistance of the mold and therefore increase its life. Moreover, friction between the casting and the mold should be reduced so that the mold can be operated at a higher casting speed.
It has been proposed to electrolytically deposit a chromium or nickel coating upon the inside surface of a mold so as to protect the mold surface against the melt in the stated manner. Layers of this kind are wear resisting and establish a low friction between the molten solidifying material in the mold, and the mold itself. It was found, however, that the manufacture of mold with rectangular or square-shaped cross section is rendered difficult for the following reasons.
A uniform coating of the corners, i.e. of the sharply curved portions of the mold is not possible because the throwing power of the electrolytic bath is relatively poor. This means that the overall thickness of the coating has to be very high, i.e. higher on the average, in order to make sure that the corners are adequately covered. However, when the protective coating exceeds about 150 micrometers, then the internal contour of the mold is changed such that casting is no longer possible. This is known in the industry as the keyhole effect.